A vacuum countergravity casting process using a porous, gas-permeable mold is described in such prior patents as the Chandley et al U.S. Pats. Nos. 4,340,108 issued July 20, 1982 and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having a porous, gas-permeable upper mold member (cope portion) and a lower mold member (drag portion) adhesively held together at a generally horizontal mold parting plane therebetween, sealing the bottom lip of a peripheral wall of a vacuum chamber to the mold such that the vacuum chamber confronts the gas permeable upper mold member, submerging the bottom side of the lower mold member in an underlying molten metal pool and evacuating the vacuum chamber to draw the molten metal through one or more ingate passages in the lower mold member and into one or more mold cavities formed between the upper and lower mold members. U.S. Pat. No. 4,340,108 seals the mold to the mouth of the vacuum chamber such that the mold parting plane lies outside the vacuum chamber. On the other hand, U.S. Pat. No. 4,632,171 seals the mold to the mouth of the vacuum chamber atop the drag portion such that the mold parting plane lies within the vacuum chamber.
U.S. Pat. No. 4,658,880 mounts the mold to the vacuum chamber by means of a plurality of reciprocable and rotatable shafts having self-tapping threads on the lower ends thereof engaging mounting sites atop the mold. Reversible motors rotate the shafts so as to screw the threads into engagement/disengagement with the mounting sites to mount/demount the mold relative to the vacuum chamber. Screwing of the threads into the mounting sites draws the mold into sealing engagement with the mouth of the vacuum chamber. Chandley, G. D. Automatic Countergravity Casting of Shell Molds, Modern Casting, Oct. 1983, pages 29-31, mounts round molds to a round vacuum chamber wherein the inside surface of the vacuum chamber includes self-tapping threads which screw into the periphery of the round mold. U.S. patent application Ser. No. 147,863 filed Jan. 25, 1988 and now abandoned describes a technique for mounting the mold to the vacuum box via a plurality of T-bar keepers engaging anchoring cavities in the mold.
When the aforesaid mold-vacuum chamber mounting arrangements are used with thin molds having more than about 400 square inches of mold confronting the vacuum chamber, there is a tendency for the molds to bow or flex inwardly of (into) the chamber when the casting vacuum is drawn in the chamber. This flexure can destroy the mold by forming cracks therein if not total fracture (i.e., implosion) of the mold into the chamber.
The aforesaid references all disclose gas-permeable molds wherein the upper and lower mold members are adhered (glued) together at the mold parting plane. The gluing process for holding the upper and lower mold members together is expensive and time consuming and elimination thereof would improve the efficiency and economics of the vacuum countergravity casting process. One technique for eliminating the need to glue the upper and lower mold members together and for resisting destructive flexure of the mold is the subject of copending U.S. patent application Ser. No. 198,229, filed May 25, 1988 of common inventorship and assignee herewith.
In copending U.S. patent application Ser. No. 198,229, a gas impermeable or permeable septum is provided in the vacuum housing and overlies the upper mold member for contacting the upper surface thereof to transmit ambient pressure to the upper mold member in opposition to the ambient pressure transmitted to the lower mold member when the mold cavities in the mold are evacuated by establishing a subambient pressure in a space between the septum and the upper mold member. A differential pressure is thereby applied on the mold in such a manner as to resist destructive mold flexure and hold the upper and lower mold members together at the mold parting plane without the need for glue therebetween. However, this technique for holding the mold members together without glue may not be amenable to certain mold configurations having numerous ingate passages extending between the mold cavities and the bottom side of the lower mold member. The presence of such numerous ingate passages may render the vacuum internal of the mold (e.g. at the parting plane) inadequate to establish a negative differential pressure on the mold sufficient to hold the lower and upper mold members together without glue.
Another technique for eliminating the need to glue the upper and lower mold members together and resisting destructive mold flexure is the subject of copending U.S. patent application Ser. No. 211,020 filed June 24, 1988 of common assignee herewith wherein spring means is provided in the vacuum chamber for pressing the upper mold member into sealing engagement with the lower mold member (sans adhesive) when the mold is mounted to the mouth of the vacuum chamber with the lower mold member sealingly engaged to the mouth.
Still other techniques to this same end (i.e., eliminating gluing of the upper and lower mold members and resisting destructive mold flexure) are the subject of copending U.S. patent application Ser. Nos. 211,023 and 211,024 filed June 24, 1988 of common assignee herewith. These techniques provide substantially rigid means for pressing the upper and lower mold members sealingly together at the mold parting plane when the mold is mounted to the mouth of the vacuum chamber.
It is an object of the present invention to provide an improved apparatus and method for the vacuum countergravity casting of molten metal including means for sealingly engaging the upper and lower mold members together at the mold parting plane by differential pressure effects (sans glue) and resisting destructive flexure of the mold after the mold is mounted to the mouth of the vacuum chamber and the vacuum chamber is evacuated. It is another object of the invention to provide such improved apparatus and method wherein the means for sealingly engaging the upper and lower mold members together without glue is useful with a wide range of mold designs including those having numerous ingate passages between the mold cavities and the bottom side of the lower mold member. These and other objects and advantages of the present invention will become more readily apparent from the detailed description which follows.